Memory elements formed from materials that can be programmed to exhibit at least two detectably distinct electrical resistivities are known in the art. One type of material that can be used as material for these programmable elements is phase-change material. Phase-change materials may be programmed between a first structural phase where the material is generally more amorphous and a second structural phase where the material is generally more crystalline. The term amorphous, as used herein, refers to a condition that is relatively structurally less ordered or more disordered than a single crystal and has a detectable characteristic, such as high electrical resistivity. The term crystalline as used herein, refers to a condition that is relatively structurally more ordered than amorphous and has lower electrical resistivity than the amorphous phase. Since programmable memory elements made with a phase-change material can be programmed to a high resistance state or a low resistance state by changing the phase of the material, one phase can be used to store a logic 0 data bit, for example, while the other is used to store a logic 1 data bit.
The concept of utilizing phase-change materials for electronic memory applications is disclosed, for example, in U.S. Pat. Nos. 3,271,591 and 3,530,441. The early phase-change materials described in the '591 and '441 patents were based on changes in local structural order. The changes in structural order were typically accompanied by atomic migration of certain species within the material. Such atomic migration between the amorphous and crystalline phases made programming energies relatively high; the electrical energy required to produce a detectable change in resistance in these materials was typically in the range of about a microjoule. This amount of energy had to be delivered to each individual memory element in a solid state matrix of rows and columns that made up a memory device. High energy requirements translated into high current carrying requirements for the address lines and for an isolation/address device associated with each discrete memory element in the memory device.
The high energy requirements needed to program the resistance of the memory elements described in the '591 and '441 patents limited their use as a direct and universal replacement for present computer memory applications, such as tape, floppy disks, magnetic or optical hard disk drives, solid state disk flash, dynamic random access memory (DRAM), static random access memory (SRAM) and socket flash memory. For example, low programming energy is important when using a plurality of programmable memory elements as electrically erasable programmable read-only memory (EEPROM), used for large-scale archival storage. Reducing the power consumption of mechanical hard drives (such as magnetic or optical hard drives) by replacement with EEPROM hard drives is of particular interest in such applications as lap-top computers because the mechanical hard disk drive is one of the largest power consumers therein. However, if the EEPROM replacement for hard drives has high programming current requirements, and consequently high power requirements, the power savings may be inconsequential or, at best, unsubstantial. Thus, programmable memory elements, in order to be used in memory devices capable of replacing a variety of conventional memory, require low programming energy.
The programming energy requirements of individual memory elements may be reduced in different ways. For example, the programming energy may be reduced by appropriate selection of the composition of the memory material. An example of a phase-change material having reduced energy requirements is described in U.S. Pat. No. 5,166,758, the disclosure of which is incorporated herein by reference. Other examples of memory materials are provided in U.S. Pat. Nos. 5,296,716, 5,414,271, 5,359,205, and 5,534,712, the disclosures of which are all incorporated herein by reference.
It has been further found that the performance of devices incorporating these memory elements are closely linked to the active volume of the phase-change material that is being addressed. Thus, the programming energy requirement may also be reduced through appropriate modification of the electrical connection whereby programming energy is delivered to the memory material. For example, a reduction in programming energy may be achieved by modifying the composition or shape of the electrical connection. Examples of such modifications are provided in U.S. Pat. Nos. 5,341,328, 5,406,509, 5,534,711, 5,536,947, 5,933,365 and RE37,259, the disclosures of which are all incorporated herein by reference.
The memory elements are generally formed in integrated circuits using sequential wafer processing. However, optimal performance and minimal programming current, and thus minimal energy, are typically obtained at dimensions for the active volume of phase-change material that fall below the minimum printable lithographic dimension. That is, using standard wafer processing techniques where the area of contact between an electrode and the phase-change material are lithographically-defined, the area of contact, and thus the active volume of phase-change material extending from that area of contact, may be larger than desired. Modification of the electrical connection, which typically involves the addition of processing steps in the formation of the memory element designed to reduce the active volume, can be complicated and add variability in the area of contact from element-to-element in a memory array including many such elements.